I've already been planning some sort of feature like this. Of course, generally battles shouldn't last long enough to require this feature, but we have some bonus levels that are just butt-grindingly long. And on a lot of those, after a while you know exactly what to do for the first 10 minutes, the challenge comes after that. What's the point of doing that same 10 minutes of stuff over and over just to fail why trying something new on the actual hard bit? You've already proven you can beat the first part, the challenge comes later.
The technical challenge here is that there's no easy way to save mid-battle with all the monsters and particles and attacks and cooldowns flying all over the screen. Trying to properly save and reload that monstrous soup of game state is just asking for neverending bugs.
So, the way to handle this is to put checkpoints or something in the middle of certain battles, and this only makes sense for sufficiently long battles.
The way it will probably work:
--One of the "waves" is not actually a wave, but a checkpoint.
--When you reach it, it stops the wave bar and gives you a chance to clear out any remaining enemies.
--Once all enemies have been defeated, the checkpoint will bookmark your position in that level.
---->This is because it's very easy to just mark where your defenders are standing, how much PSI you have, etc.
---->It may or may not make sense to refresh your dudes to full health / cooldown at this point (but not refresh your mcguffin's PSI)
--From then on, you can reset the battle from the start (and lose the checkpoint), or reset from the checkpoint
--If you quit the battle, your checkpoint is preserved until you start a different battle or change your party configuration somehow.
--If you go back to the battle, you can jump back in at your checkpoint, or start from scratch (erasing the checkpoint).
That's what I'm thinking for now.
And I don't really care about save-scumming / abuse. Hero Mode is easy enough to cheat at (as are all the achievements), hell we even have the free 300%, no-penalty XP option, and plenty of people still feel bound by honor to not cheat.
Basically, what it comes down to, is am I creating a game that is fun and challenging, without optimal strategies that are painful? For instance, that's why we don't have any minigames where you have to manually pick up coins or whatever with the mouse. It's tedious and not fun, but if you want to win you'll do it and you resent having to. So I want to fix the very real problem of longer battles that force you to replay the same bits over and over.